[CORE-14950] ct: skip oauth tests in FIPS mode

[dotnwat]

Infra issues prevent cert creation for these tests. See JIRA ticket for more details.

Backports Required

• [x] none - not a bug fix
• [ ] none - this is a backport
• [ ] none - issue does not exist in previous branches
• [ ] none - papercut/not impactful enough to backport
• [ ] v25.3.x
• [ ] v25.2.x
• [ ] v25.1.x

Release Notes

• none

[vbotbuildovich]

CI test results

test results on build#77623

test_class	test_method	test_arguments	test_kind	job_url	test_status	passed	reason	test_history
PartitionReassignmentsTest	test_reassignments_cancel	null	integration	https://buildkite.com/redpanda/redpanda/builds/77623#019b05b3-aa77-4d64-986d-4754c8ff6702	FLAKY	9/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0812, p0=0.5714, reject_threshold=0.0100. adj_baseline=0.2244, p1=0.3066, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=PartitionReassignmentsTest&test_method=test_reassignments_cancel